The current required by an active electronic system is constantly in flux. Notwithstanding these current fluctuations, the power supply of an electronic system needs to provide a stable output voltage. Without a stable supply voltage digital circuitry will suffer from bit errors and analog circuitry performance will be degraded by bias point shifting. This requirement has been present in electronics circuits since their inception and is solved through the use of voltage regulators. A voltage regulator receives a supply voltage and outputs a regulated voltage to an electronic circuit. The regulated circuit is referred to as the load of the voltage regulator. Modern integrated circuits such as those found in computers and mobile phones commonly utilize linear voltage regulators. The use of linear voltage regulators is the favored method in integrated circuit applications because they provide a clean voltage supply, meaning that the regulated voltage is relatively free from noise.
There is a large body of prior art related to linear voltage regulators. A generalized diagram of a regulator circuit can be found in FIG. 1. Circuit 100 is an example of a positive voltage regulator since the regulated voltage Vout has a lower potential than the supply voltage Vin. Negative voltage regulators provide for the opposite relationship between regulated and unregulated voltage. In a linear voltage regulator pass element 104 is an active device, such as a bipolar junction transistor (BJT) or metal-oxide semiconductor (MOS) field effect transistor, connected between the unregulated supply voltage and the regulated output voltage. The active device is controlled by a feedback path comprised of feedback network 103 and amplifier 102. The feedback network is most commonly a resistor divider network made up of resistors such as 105 and 106. In voltage regulators, amplifier 102 is commonly referred to as the error amplifier and is biased by a voltage that has a functional relationship to the supply such as that produced by reference generator 101. As typical in a negative feedback system, fluctuations in the current through feedback network 103 results in a counteracting change in the current supplied by pass element 104. In this manner, the voltage at Vout remains constant independent of the current drawn from Vin.
The prevalence of portable electronics has placed increasingly restrictive demands on the performance of low power voltage regulators. In the interest of user convenience, portable electronics need to be designed for optimal power consumption to preserve battery life. One method of preserving power in an electronic system is to shift the system into different phases of power consumption depending upon the systems varying functionality requirements. This helps to preserve power due to the inverse relationship between functionality and power consumption in electronic circuits. The ratio of the variant phase currents, such as the operating current and the standby current, will increase as circuit performance is optimized. Low power consumption also requires a rapid transition between these phases. Fast transitions are desirable because a circuit in transition is dissipating more power than it does in a dormant state, but it is not yet accomplishing anything constructive for the circuit's operation.
The fast transition times required by modern circuits require rapid charge delivery in response to rapid changes in the regulated load. A bypass capacitor can deliver charge in response to very high frequency perturbations in the regulated load but is limited in the amount of charge it can supply. A linear voltage regulator can deliver charge in a sustainable and controlled manner. However, the speed of a linear voltage regulator is inherently limited by the stability requirement of its incorporated feedback loop. This is because without proper compensation a linear voltage regulator can suffer from instability and compensation limits the bandwidth of voltage regulators. The bandwidth must be limited because at high frequencies the characteristic of the loop will change from negative to positive feedback. If high frequency signals are amplified by positive feedback the circuit will become unstable and will be ineffective as a regulator. This property is one of the main drawbacks of linear voltage regulators. The most common substitute for linear voltage regulators are DC-DC switching regulators. This type of regulator also suffers from instability and tight speed constraints in that the switching speed needs to be around five times the bandwidth of the regulator. Circuits that combine the characteristics of DC-DC switching regulators and linear voltage regulators such as U.S. Pat. No. 5,309,082 to Pyane or U.S. Pat. No. 7,167,054 to Dening are not meant to alleviate the speed and power constraint but instead address the issues of power dissipation across the pass element and the high cost and poor regulation of DC-DC switching regulators.
A notable solution for the limited bandwidth problem of linear voltage regulators focuses on providing high frequency compensation that is not controlled by feedback. Such a circuit is described in U.S. Pat. No. 6,809,504 to Tang. An open loop system does not have frequency dependent stability constraints and therefore can operate at frequencies that exceed the requirements of linear voltage regulators. The implementation in the Tang circuit comprises pulse generators that input a set current to the load for a set time in response to a rapid change in the regulated current. The objective of such a circuit is for the predetermined current provided by the pulse to cancel the transient current that the slow linear regulator cannot track. The advantage of this approach is that the circuit can operate at very high frequency as there is no stability limitation on a system that does not have a feedback path. This approach carries a related disadvantage in that the open loop approach cannot measure and apply the exact current required. In some cases the predetermined compensation current may be so far from the desired current that the slow linear voltage regulator would have provided a better estimate on it own.
A method applied in the field of phase-locked loops to improve the transient response of a system loop utilizes both digital and analog closed loop filters. Such phase-locked loop architectures are called hybrids. An example of such architecture can be found in U.S. Pat. No. 5,978,425 to Takla. The general purpose of the phased-locked loop is to match up the clock scheme of the input signal to the clock utilized to receive the input signal. The approach utilizes a digital loop to provide a fast, though course, adjustment of the loop during a calibration phase. After calibration, a slower analog loop provides high resolution and accuracy. If this approach is not utilized the startup time for a phase locked loop could be extremely large. Control circuitry is necessary to determine when the digital loop has served its purpose and the analog loop can take over. Such a system utilizes the fast settling of a digital loop and the accuracy of an analog loop as complements to avoid the drawbacks and enhance the benefits of the respective loops.
The approach of utilizing analog and digital loops in tandem to improve the transient response of an electronic system can be applied to voltage regulators. An example of such a solution is developed further in the previously mentioned patent to Tang and is fully described in U.S. Pat. No. 6,975,494 also to Tang. This circuit incorporates a typical linear voltage regulator and augments its performance by adding independent discrete current sources controlled by voltage sensing circuitry on the load. The linear feedback circuit operates by voltage sensing changes in the regulated voltage through a feedback system. Likewise, the added circuitry functions by voltage sensing changes in the circuit and applying a set amount of current in response to the sensed voltage passing specific thresholds. The range in between these thresholds over which the discrete current boosting is not activated is called the dead zone. Unlike the open loop system this portion of the circuit can be subject to instability and oscillations due to its utilization of sensing and feedback.
An embodiment of the Tang approach is circuit 200 illustrated in FIG. 2. The circuit utilizes typical voltage regulator 201 that regulates the voltage at node Vout using an analog feedback loop which is stabilized by capacitor 203. The circuit also uses an independent discrete current source in a nonlinear feedback loop comprised of compare and control circuitry 206, voltage transform circuitry 202 and discrete current sources 208 and 207. The discrete current sources are not active for a range of voltages around the targeted regulated voltage at node Vout. The passive components required for stability are inductor 204 and storage capacitor 205. For rapid variations in the current to load 209 discrete current sources 207 and 208 will activate and either source or sink current. However, during stable operation the nonlinear feedback loop is functioning in the dead zone and only the linear analog loop in regulator 201 will be active, while current sources 207 and 208 will be off. The span of Vout for which the discrete nonlinear feedback loop is inactive is called the dead zone and is set by the voltages delivered to nodes Vr+Δ1 and Vr+Δ2.